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Ponyboy7 -> RE: OK,is Glass a .................. (10/4/2007 5:41:39 PM)
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quote:
ORIGINAL: samboct
Ponyboy
I suspect one of the causes of disagreement is that glass transition is not well defined. About.com had a nice simple plot that reminded me of the exam I took on the subject. Basically the volume change of a glass with temperature is altered above the glass transition temperature when compared to the volume change with temperature below the glass transition temperature. Above the glass transition temperature, the slope of the volume change is steeper than below. These are both straight lines- so the point where this changes is discontinuous.
Your comment about time dependence and cooling is well taken- since the properties of the material can change depending on how rapidly you cool- which can also change the calculated glass transition temperature. I'm not sure the same thing applies to heating though since you're breaking bonds, not forming them. Again, this is to the point of hysteresis- things happen differently with a glass depending on how you get there.
Physicists and chemists often use different definitions of liquid and solid. Chemists are also perfectly happy classifying things as liquids (no long range order greater than 1 molecule away) polymers (some longer range order- several molecules away to 50 molecules away) goopy liquid to apparently solid, or crystalline- long range order to the surface- classical solids. Physicists bang on stuff to figure out if it's solid or not, whereas chemists play with temperature a bit more.
I suspect that glass flow with time may again be dependent on the type of glass being discussed- hasn't anybody had a piece of glass warp on them? What do you call that if not flowing?
Sam
Hi Sam,
Sorry about quoting the whole thing, but this way I don't have keep scrolling back and forth. Anyway, I will agree that physcists and chemists do have slightly different definitions. I'm sure if we put a physicist, a chemist and a biologist in a room we'd all have different definitions (if you add a mathematician to the room then it's the start of a joke I read :)). Anyway, the glass transition point has become quite well defined fairly recently; there was a recent article on this in Physical Review Letters, if you're interested; they did a neat experiment using neutron diffraction. The glass transition temperature (GTT) is a function of rate and temperature, and if we keep that in mind, then using differential scanning calorimetry we can get very precise values for GTT. However, nearly all phase changes do have a characterisitc discontinuity in the nth derivative of the free energy.
What you are saying is certainly true, however, this is for an idealized glass. An idealized glass will not exhibit a phase transition, you are correct, but for real glasses one does exist. One of the characterisitics of a glass transition, probably more of a physics one, is symmetry breaking. This is how we can more easily define when the transition takes place. As you know, symmetry breaking is characterisitc of most phase transitions, but this characterisitc is not usually used because simpler definitions will suffice for most materials. Of course symmetry breaking at the GTT is quite different from symmetry breaking at, say, a crystlline solid's melting point. However, symmetry is broken; whereas a crystalline solid breaks atomic symmetry at its mp (glass cannot do this since it has no long range order, only nearest neighbor, or short range order, and so has no atomic symmetry to break). Glass does, however, have bond symmetry, or, more precisely, bond order symmetry. This symmetry does break via dimensionality (this is technically also true for crystalline solids at breakdown of hyperscaling). I think this is a very good defintion of a phase change for glass from solid to liquid because it offers a very precise point for the transition.
As for antique windowpanes "[...W]hy are the panes of antique window glass thicker on the bottom than the top? There really are observable variations in thickness, although there have been no statistical studies that document the frequency and magnitudes of such variations. This author believes that the correct explanation lies in the process by which window panes were manufactured at that time: the Crown glass process. " (Plumb et al). Moreover, warping occurs in many solids aside from glass; just because glass may warp easier, than say, titanium, does not make it a liquid.
I think the crux of the issue here is being able to properly characterize the transition point. Many of the classical definitions do break down, so if we use those we will get non-sensical results. The good thing is the classical definitions were made when we had a lesser understanding of what a phase transition really is; I'm not saying we know everything yet, we definately do not, but we do know enough to find a precise point (given certain conditions) where the phase transition takes place. I think it all makes perfect sense if we talk about derivatives of free energy or symmetry, rather than visual observations, which can get in the way and be confusing. The GTT is a multi variable function, but glass does make a clear transition from solid to liquid at a given GTT.
Anyway, I will certainly agree that there is a discontinuity in volume change vs. temperature for a glass, but again, I think we should look at the more modern definitions of solids, liquids, and phase transitions. I think once we do this, most of the confusion is eliminated.
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